- * f c * -6 9 -.I. lcal '. DEVELOPMENT AND TESTSNG OF A SMALL MODERATE TEMPERATURE GEOTHERMAL SYSTEM *.BY Roger F. Harrison Terra Tek, Inc. Salt Lake City, Utah C; K. Blair Terra Tek, Inc. - Salt Lake City, (Ibh David 5. Chapman University of Utah Salt Lake City, Utah - ABSTRACT - Geological and geophysical exploration of the Monroe-Red Hill geothermal system in southern Utah has been followed by the drilling of two intermediate depth test holes and the drilling and testing f.a 457 meter production well. The test holes confirmed that the hot water system was structurally controlled by the steeply dipping Sevier Fault and suggested that the exploitable reservoir temperature was about 74OC. The production well intersected the top of the Sevier Fault zone at about 35 meters and was successfully completed to 457 meters. Preliminary testing of the production well indicated a reservoir with acceptable storape but somewhat limited permeability. BACKGROUND The Monroe-Red Hill geothermal system is situated on the Sevier Fault near Monroe, Utah (see Figure 1). The proximity of the geothermal system to the town of Monroe, population 2,, presents an ideal situation for direct heat app?ication if the resource base can be proven sufficient. Geological and geophysical exploration of th eothemal system (Miller, 1976; Mase, 1978; Kilty, 1978; Chapman, et.a?., 1978) has resulted in the following infomation: (1) the hot springs and geothermal system are structurally controlled by the Sevier Fault which separstes upthrown Tertiary Volcanics to the east from Quanternary alluvium to the west; (2) the system is effectively mapped by dipole-dipole resistivity with low resistivities of 3 ohmmeters coinciding with discharge areas; (3) temperature-depth profiles show a pattern consistent with a model of hot water discharge essentially confined to the Sevier Fault zone which dips at an angle of about 7' to the west; and (4) the totdl conductive and convectiv heat loss for the system is 8 megawatts. Y In order to further define the structural controls of the anomaly, two 6-3/4 inch diameter test holes, designated MC 1 and MC 2, were drilled to 11 meters and 251 meters, respectively. The well lithologies, indicated in each case a sinter deposit in the shallow subsurface, a1 tered Quaternary alluvium consisting primarily of quartz latite fragments derived from the volcanic range to the east, and at the base more consolidated volcani: bedrock. The alluvium bedrock interface in each case was delineated by a change in the drilling rate, and the caliper, natural gama, and lithology logs. -125-..
1-126- Lp? t s ~ffceelltuted PALEOZOIC E3 SEDIYEfITARY ROCKS. WATERNARY ALUNUI YAPPPED FAU.1 I DIIWED WRC HFERRm 1 cr INDIFfERENTIATED TERTIARY SEDIHENTARY ROCKS Figure 1. Location and General Geology of the Monroe-Red Hill Geothermal System. A dip of 67" 2 3" for the Sevier Fault zone was computed on the basis of the intersections with more consolidated volcanics, the well geometries, and an assumed fault trend of north 1" west. This dip is shallower than that deduced from the geophysical modeling and may indicate step faulting. Temperatures observed in MC 1 and t.lc 2 and the thermal gradient holes M1-M4 shown in Figure 2, indicate a strong convective system rigorously confined along the fad t zone. PRODUCTION TEST WELL. Strong artesian flows from fractures in the footwall of the fault at 229 meters led to the. conclusion that the sinter deposits at the surface were capa potentially productive resource driven by a strong hydraulic gradient. Penetration of fractures. thereby inducing production from depth was the principle goal of the production test drilling. Flow rates up to 4 liters per second would be required to service the planned district heating scheme, and therefore, a large diameter surface casing would be necessary to accommodate # ping I: 6
-127-1EMPtRAlUlit 1%) Figure 2. 1 Temperature Profiles in Thermal Gradient and Test Holes. the appropriate pump should the productivity of the well prove to be adequate. The well was designed and located on the basis of the geometry inferred from - the test hole drilling to intersect the volcanic bedrock at 274-35 meters. Alluvium was anticipated above this point. Major production was expected from fractures in the bedrock. It was felt that production fromthe 35 meter plus level offered the advantages of good artesian flows due to increased driving potential with depth, the possibility of temperatures exceeding 8 C which would be of considerable benefit in view of retrofit requirements for existing building heating systems, and minimum influence on surface hydrologic conditions. An unexpected lacustrine formation was encountered at 27 meters. This generally hard limestone formation with interbedded narrow clay zones persisted to approximately 35 meters. A gradual increase in the fraction of volcanic material in the drilling returns from 32 to 35 meters was interpreted as the intersection of a brecciated zone at the volcanic s interface. The transfer between this region and more consolidated bedrock was not immediately evident in the drill rate or caliper logs and was difficult to identify in the lithology logs. The upper portion of the hole was reamed and 16-inch casing whs cemented to 68 feet. The total drilled depth at this time was 4 meters and after cleaning, the well produced an artesian flow rate of approximately 6 liters per second. The lithology logs indicated that the well had penetrated at least 46 meters of bedrock,. however, no significant lost circulation had been encountered or fracture zones identified in the caliper or temperature logs. The well was deepened to 457 meters in an effort to intersect the water carrying fractures which had been the original target.
-128-. WELL TESTING RESULTS Results of a 7-hour drawdown test during which the well was pumped at 2.5 liters per second are shown in Figure 3. The two monitor wells MC 1 and MC 2 4 are 5 meters and 11 meters from the production well respectively. Computed storage coefficients indicate that confinement increases with depth. This result is consistent with the presence of a discharging hot spring zone. Simple * Theis (1935) type curve analysis of the data gives transmissivities of 2.5 rn2/hr and 1.7 M'/hr for WC 1 and EIC 2 respectively. The large initial drawdown in the production well is due in part to formation damage near the wellbore. The leveling off in drawdown after approximately 12 minutes indicates the presence of a high permeability or recharge zone, perhaps a water carrying fracture, at between 12 and 2 meters from the wellbore.,. The well was surged in an effort to increase the well efficiency. The results of a subsequent drawdown test in which the well was pumped at 38 liters per second for 3 hours are shown in Figure 4. Apparent transmissivities of 2.3 M2/hr and 1.5 M2/hr in MC 1 and MC 2 are consistent with results from the previous test.. The drawdown results from the production well however, do not indicate a strong recharge zone. The complicated geometry of the aquifer obviously precludes the use of the simple Theis analysis. The two shallow monitor wells MC 1 and MC 2 probably exhibit the characteristics of the alluvium in the shallow subsurface. The production well draws both from storage in the alluvium and directly from the brecciated fault zone which carries water to the surface.. 9 4.1.1 1 Figure 3. Drawdown Data for 7-Hour Pump Test.
-129- DI. I I IO I1 1 9 I MONITOR WELL( MC O O O O O e Figure 4. Drawdown Data for 3-Hour Pump Test. REFERENCES Miller, C. D., 1976, Alteration and Geochemistry of the Monroe Known Geothermal Resource Arear M.S. Thesis, University of Utah. Rase, C. W., 1978, Geophysical Study of the Monroe-Red Hill Geothermal System, M.S. Thesis, University of Utah. Kilty, K. T., 1978, Forced Convective Heat Transfer, M.S. Thesis, University of Utah. Chapman, D. S., Kilty, K. T., and blase, C. W., 1978, Temperatures and their Dependence on Ground-Water FLOW in Shallow Goethermal Systems, Geothermal Resources Council, Transactions Vol. 2, p. 79-82. Theis, C. V., 1935, The Relation Between the Lowering of the Piezometric Surface and the Rate and Duration of Discharge of a Well Using Ground \later Storage. Trans. Am. Geophysical Union, pp. 519-524, Washington. ACKNOWLEDGEMENT The authors would like to thank Hr. Kip Smith of the University of Utah Research Institute, Earth Scionccs LaSoratory, for his usistance in revlewlng and analyzing the well test data.